3. Description of the wind speed probabilities graphical products

Each wind speed probabilities graphic provides probabilities (in percent) that wind speeds of at least 34 kt (39 mph, tropical storm force), 50 kt (58 mph), or 64 kt (74 mph, hurricane force) will occur during cumulative time periods at each specific point on the map. The cumulative periods extend into the 5-day forecast period at 12-hour intervals (that is, 0-12 h, 0-24 h, 0-36 h, ... , 0-120 h). An individual graphic is produced for each active tropical cyclone for each cumulative time period and for each wind speed threshold. The capability to animate through the periods is also provided. These cumulative probabilities indicate the overall chances that the indicated wind speed will occur at any specific location on the map during the period between hour 0 and the forecast hour.

In other words, these cumulative probabilities tell decision-makers the chances that the event will happen at any point on the map within the time period stated on each graphic.

The example graphic below (an earlier experimental version) shows the hurricane force (64 kt, 74 mph) wind speed probabilities for Hurricane Charley (2004), based on advisory 14 issued at 5:00 PM Thursday August 12, the day before landfall in southwestern Florida. When this advisory was issued, the hurricane warning was extended northward along the west coast of Florida from Bonita Beach northward to Bayport (to include Fort Myers, Port Charlotte, Sarasota, and Tampa). The cumulative 0-120 hour values are shown here to indicate the overall chances of experiencing hurricane-force winds at any point on the map. While the exact official track forecast for this advisory goes over Tampa, it is clear from this graphic that the chances of experiencing hurricane-force winds from this event are nearly the same (about 30%) over a large portion of the coastline and over many inland areas, including the eventual landfall location at Port Charlotte, FL. This graphic is an excellent example of a situation in which the wind speed probabilities can help users to understand forecast uncertainties, such that they are not surprised by any relatively small changes in the track. This graphic also shows why it is crucial to make proper preparations when a watch or warning is issued for your area, even if the exact track forecast does not go over your area.

An example of a suite of 5-day wind speed probabilities graphics for an advisory package is provided below with explanations that follow. Examples are provided for 34 kt (39 mph, tropical storm force) or greater, 50 kt (58 mph) or greater, and 64 kt (74 mph, hurricane force) or greater.

Fig. 2 Wind speed probabilities graphics for Hurricane Katrina (2005) advisory #14. Graphics show cumulative probabilities of wind speeds of at least 34 kt (39 mph, tropical storm force) occurring at any point on the map during the 5-day period beginning 2:00 PM EDT August 26.

Fig. 3 Wind speed probabilities graphics for Hurricane Katrina (2005) advisory #14. Graphics show cumulative probabilities of wind speeds of at least 50 kt (58 mph) occurring at any point on the map during the 5-day period beginning 2:00 PM EDT August 26.

Fig. 4 Wind speed probabilities graphics for Hurricane Katrina (2005) advisory #14. Graphics show cumulative probabilities of wind speeds of at least 64 kt (74 mph, hurricane force) occurring at any point on the map during the 5-day period beginning 2:00 PM EDT August 26.

The example set of probabilities graphics above are based on Hurricane Katrina (2005) advisory number 14, which was issued 18 hours prior to a hurricane watch being issued for southeastern Louisiana and about two and a half days prior to the initial landfall of the center of the hurricane in southeastern Louisiana.

The probabilities show that the hurricane is not a point and has the potential to affect a large area. The probabilities result from uncertainty in the forecast of track, intensity, and size. The Katrina example involves a large and intense hurricane. However, not all storms will produce probabilities this large over as wide an area.

The hurricane-force (64 kt, 74 mph) probabilities are smaller than for tropical storm force (34 kt, 39 mph), primarily because hurricane-force winds do not extend as far out from the center of the hurricane. Note in this example that the probabilities of hurricane-force winds through 5 days (120 hours) are essentially the same for southeastern Louisiana and the western Florida panhandle. Therefore, even though the exact track forecast might go over a particular location on the coastline, there are many other locations that have the same chance of experiencing hurricane conditions during the event. It is important to understand that the probabilities are for specific points on the map. The chances that hurricane-force winds will occur somewhere along the Gulf coast in this Katrina case are much larger than the chances at any one point. In other words, in advance of the landfall of Katrina, it was nearly certain that portions of the northern Gulf coast would be impacted by hurricane-force winds, but it was not certainly exactly who would experience those winds. As it turned out, portions of southeastern Louisiana did experience sustained hurricane-force winds, while the western Florida panhandle did not.

In the above example, the hurricane-force wind speed probabilities are relatively small (less than 25%) in magnitude at points along the northern Gulf of Mexico coastline through the 5-day forecast period. It is important for users to realize that probabilities that may seem relatively small may still be quite significant. The probabilities may indicate there is a chance that a damaging or even an extreme event may occur at your location. As a storm gets closer to land, relatively small probabilities may warrant making preparations to protect lives and property. Users are urged to consider the potentially immense cost (in terms of lives, property, etc.) of not preparing for an extreme event, even if the chances at an individual point are only perhaps 1 in 20 (5%) or 1 in 10 (10%) that the event will occur.

While separate graphics are centered on each individual tropical cyclone, probabilities resulting from more than one active tropical cyclone may be seen on each graphic (example below). The tropical cyclone on which the graphic is centered is labeled with a diamond, which represents the location of the center of the tropical cyclone at the beginning of the forecast period.

Fig. 5 Wind speed probabilities graphics for Hurricane Helene (2006) advisory #17. Graphics show probabilities of wind speeds of at least 50 kt (58 mph) occurring at any point on the map during the 5-day period beginning 8 AM AST September 16. Note that Hurricane Gordon, located to the north of Hurricane Helene, is generating additional wind speed probabilities on this graphic.

Landfall intensity, however, cannot be inferred from the table. The incorporation of track error and its impact on the intensity probabilities is the reason that using the intensity probabilities to infer the intensity of a cyclone at the time of landfall can be misleading. To illustrate this point, let's examine advisory 14 for Hurricane Katrina issued at 5 PM EDT 26 August. In this example, the official forecast shows Katrina making landfall along the northern Gulf coast just after the 72 hour forecast time (Figure 7) as a major (category 4) hurricane. The track errors incorporated into the probability technique account for the possibility that Katrina may already be inland by the 72 hour forecast time. This results in a decreasing probability that the cyclone would be a major hurricane at 72 hour, but that would not accurately reflect the probability that the cyclone would be a major hurricane at the time of landfall. For this advisory, the maximum intensity probabilities for the various hurricane Saffir-Simpson categories were 14, 12, 19, 12, and 3 percent for categories 1 through 5, respectively. While the actual forecast was for a category 4 hurricane near landfall, the maximum wind speed probabilities at the 72-hour forecast time from this advisory indicated nearly equal chances for Katrina to be category 1 through 4 (12-19%). Since the algorithm take land into account, the maximum intensity speed probabilities for cyclones near land tend to be spread nearly equally among the different categories in the table. In this case, Katrina made landfall as a category three storm in southeast Louisiana, but at the verifying 72-h forecast time (1800 UTC 29 August) from this example, Katrina had moved inland and weakened to a category 1 storm.

It is hoped that future work will result in the capability of NHC providing landfall intensity (maximum sustained wind) probabilities.